AD9913 profile switching issue

Hello,

  I have noticed that when selecting a profile using the AD9913 direct switch mode both the frequency and the phase of the DDS are changed no matter what value I use for the destination bits in CFR1[13:12].  For example, I set the 8 profiles using the following frequency and phase values:

Profile Freq. (MHz) Phase (degrees)
0 10.0 0.0
1 11.0 10
2 12.0 20
3 13.0 30
4 14.0 40
5 15.0 50
6 16.0 60
7 17.0 70

Initially, the DDS is operating in single tone mode with a freq. of 10 MHz and a phase offset of zero degrees.  I now set the following value for CFR1:

0x0841_1000. (in binary: 0b0000_1000_0100_0001_0001_0000_0000_0000) indicating the following:

  • CFR1[27] = 0b1 (profiles controlled by CFR1[22:20])
  • CFR1[22:20] = 0b100 (select profile 4)
  • CFR1[16] = 0b1 (direct switch mode active)
  • CFR1[13:12] = 0b01 (destination is phase word)

The output of the DDS now shows a frequency of 14 MHz.  It seems to me that with the above CFR1 setting only the phase word should be loaded and the DDS output frequency should not change.

If I repeat the above operation using the following profile settings in the table below I confirm from the DDS output that the phase word from profile 4 is successfully loaded.  (Presumably the frequency tuning word is also loaded but I deliberately set all the profile frequencies to 10 MHz in order to check that the phase word was being successfully loaded.)

Profile Freq. (MHz) Phase (degrees)
0 10.0 0.0
1 10.0 10
2 10.0 20
3 10.0 30
4 10.0 40
5 10.0 50
6 10.0 60
7 10.0 70

I am somewhat puzzled by this behaviour.  Am I missing something?

Thanks, Tim M.